sun50i: a64: Add Olimex A64-Teres-I board initial support

Olimex A64-Teres-I board is a mainboard (the only one so far)
for Olimex Teres-I DIY laptop kit.

Key features:
- Allwinner A64 Cortex-A53
- Mali-400MP2 GPU
- AXP803 PMIC
- 2GB DDR3 RAM
- MicroSD Slot
- 16GB eMMC Flash
- eDP LCD display
- HDMI
- USB Host
- Battery management
- 5V DC power supply
- Certified Open Source Hardware (OSHW)

Works:
- i2C
- MMC/SD
- PWM backlight

Known broken:
- Internal keyboard (seems to be because the keyboard firmware loads a
bootloader first, and then disconnects bootloader and connect real
keyboard). External ones connected to the USB port work fine.

This patch enables support for the A64-Teres-I board to u-boot,
including enabling screen backlight (lacking from Linux device-tree).

Linux commit details about the sun50i-a64-teres-i.dts sync:
"arm64: dts: allwinner: a64: Rename uart0_pins_a label to uart0_pb_pins"
(sha1: d91ebb95b96c8840932dc3a10c9f243712555467)

Cosmetic warnings regarding whitespace and placement of SPDX notice for
dts file was ignored.

config and .dtsi file are adapted from pinebook files.

Tested-by: Jonas Smedegaard <dr@jones.dk>
Signed-off-by: Jonas Smedegaard <dr@jones.dk>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
[jagan: move board entry in MAINTAINERS file at proper position]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>